Reid's defense of Jaczko, a former Reid aide, went far beyond statements of praise and included a sharp critique of the four NRC commissioners -- including two Democrats.

"It is sad to see those who would place the interests of a single industry over the safety of the American people wage a politically-motivated witch hunt against a man with a proven track record of ensuring that nuclear power is produced as safely and responsibly as possible," Reid's office said in a statement Saturday.

The four NRC commissioners said in a letter to the White House that they have "grave concerns" about Jaczko. They said his bullying style is "causing serious damage" to the commission and creating a "chilled work environment at the NRC."

The letter was written Oct. 13 but was made public late Friday. It stops short of calling for the chairman to resign, but says Jaczko's actions could adversely affect the agency's mission to protect health and safety at the nation's 104 commercial nuclear reactors.

Among other claims, the letter says Jaczko "intimidated and bullied" senior career staff, ordered staff to withhold information and ignored the will of the panel's majority. The letter was signed by Democrats William Magwood and George Apostolakis, as well as Republicans Kristine Svinicki and William Ostendorff.

Jaczko, in a detailed response also sent to the White House, said problems at the agency were not his fault but instead stem from "lack of understanding" on the part of the other four commissioners.

Rep. John Shimkus, R-Ill., said Saturday that Jaczko should be fired.

Shimkus, who chairs an Energy and Commerce subcommittee on environment and the economy, said President Barack Obama "has a responsibility to correct deficiencies in the executive branch -- and obviously this is a clear deficiency." Shimkus led a hearing this spring that centered on Jaczko's leadership style, and complaints that he is autocratic and ignores his fellow commissioners.

"I would have thought that would have given (Jaczko) an opportunity to kind of turn things around. It seems like things got worse, not better," Shimkus said.

Boxer, who chairs the Senate Environment and Public Works Committee, praised Jaczko for a "swift and effective response" to Japan's nuclear crisis and said the NRC commissioners should support Jaczko "as he translates the lessons of Fukushima into an action plan that will make America's nuclear plants the safest in the world."

Rep. Edward Markey, D-Mass., also backed Jaczko. Late Friday, Markey made public a 23-page report accusing the four NRC commissioners of trying to impede U.S. nuclear safety reviews after the Japan crisis.

"Instead of doing what they have been sworn to do, these four commissioners have attempted a coup on the chairman and have abdicated their responsibility to the American public to assure the safety of America's nuclear industry," said Markey, a longtime nuclear critic.

Jaczko was an aide to Markey before joining Reid's staff.

The dispute comes after an inspector general's report released in June exposed long-simmering internal strife under Jaczko. In August, Republican senators asked the inspector general to investigate whether Jaczko had authority to declare the Japan nuclear crisis an emergency -- which grants him additional powers -- since the crisis occurred on foreign soil.